Experimental determination of aluminous clinopyroxene–melt partition coefficients for potassic liquids, with application to the evolution of the Roman province potassic magmas

Abstract

We have experimentally determined the partition coefficients of rare earth elements (REE), HFSE (Ti, Zr, Hf, Nb, Ta) and other trace elements between clinopyroxene and potassic silicate melts at 0.1–200 MPa and 1042–1140°C. The major and trace element contents of clinopyroxene and liquid were determined by electron probe and ion microprobe, respectively. The liquidus clinopyroxenes are extremely rich in Al 2O 3 (up to 12%) and Fe 2O 3, which enables us to test suggestions that partition coefficients of HFSE and REE depend on the Al content of the clinopyroxene (e.g., Lundstrom, C.C., Shaw, H.F., Ryerson, F.J., Williams, Q., Gill, J., 1998. Crystal chemical control of clinopyroxene–melt partitioning in the Di–Ab–An system: implications for elemental fractionations in the depleted mantle. Geochim. Cosmochim. Acta 62, 2849–2862.). When compared with previous data for low-alumina clinopyroxene crystallised under similar P– T conditions, the increase in weight partition coefficient D i (=[ I] xtl/[ I] liq) for all the HFSE and REE is dramatic. For Ti and the REE, partition coefficients increase by about one order of magnitude as Al iv concentration of clinopyroxene increases from 0.02 to 0.5. For Nb, Ta and Zr, the effect is even more pronounced, a two order of magnitude increase being found to correspond to this increase in Al iv. We applied the data to model the fractionation of historic lavas from Vesuvius. Previous work using the major elements (Belkin, H.E., Kilburn, C.R.J., DeVivo, B., 1993. Sampling and major element chemistry of the recent (AD 1631–1944) Vesuvius activity. J. Volcanol. Geotherm. Res. 58, 273–290.) indicates a liquid line of descent from trachybasalt to leucite tephrite dominated by clinopyroxene crystallisation. The REE and HFSE concentrations in the lavas are consistent with 40–50% fractional crystallisation of clinopyroxene (accompanied by minor leucite) between the two magmatic endmembers, with partition coefficients for the REE appropriate for clinopyroxenes containing ≤7.5 wt.% Al 2O 3. Pyroxenes of the latter composition are frequent phenocrysts in the lavas and previous experimental work (Dolfi, D., Trigila, R., 1988. Chemical relations between clinopyroxenes and coexisting glasses obtained from melting experiments on alkaline basic lavas. Rend. Soc. Ital. Mineral. Petrol. 43, 1101–1110; Trigila, R., De Benedetti, A.A., 1993. Petrogenesis of Vesuvius historical lavas constrained by Pearce element ratio analysis and experimental phase equilibria. J. Volcanol. Geotherm. Res. 58, 315–343.) implies that they precipitate under very low pressure low water activity conditions, i.e. within the magma chamber at <200 MPa.